This specification relates to simulating subterranean rock blocks in an injection treatment simulation. During a fracture treatment, fluids are pumped under high pressure into a rock formation through a well bore to fracture the formation and increase permeability and production from the formation. Applied mathematical models can be used to numerically simulate certain aspects of a fracture treatment. For example, discontinuous deformation analysis (DDA) can be used to simulate rock deformation, rock stress, and fracture propagation.